Short answer: like most Italians, I use the alveolar trill /r/ (geminate or not).<rr> is just like in Spanish, while a single <r> is a single, "one-contact" trill. However, here are a few remarks for a more elaborate answer:

- some texts classify what I called a single trill as a "tap/trill" (perhaps suggesting that both realizations are valid – I am confused now)- the third paragraph of this document (which focuses on a very specific subject that is beyond our scope, but this one paragraph talks about Standard Italian) suggests that some scholars consider a third realization of <r>, too, besides the two trills: the tap- I believe that some speakers (including but not limited to the Veneto region) do use the tap /ɾ/ with a certain consistency, for example in inter-vocalic position

Well for some reason words starting with r in Vietnamese can be pronounced as a retroflex fricative [ʐ], a postalveolar fricative [ʒ], a flap [ɾ], a trill [r], a fricative flap [ɾ̝,] or a fricative trill [r̝] - oh and also as an alveolar approximant <ɹ> and lazily as ɣ. I don't like uvular approximant [ʁ] very much...but I do love them trills!